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NDT Materials and Processes > Mechanical Properties > Flashcards

Mechanical Properties Flashcards

1
Q

What are mechanical properties?

A

those properties that involve a reaction to an applied load

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2
Q

What are the most common mechanical properties?

A

Strength, ductility, hardness, impact resistance, and fracture toughness

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3
Q

Are most structural materials isotropic or anisotropic?

A

Anisotropic

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4
Q

What does anisotropic mean?

A

The material properties vary with orientation

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5
Q

Do mechanical properties depend on the product form?

A

Yes. Sheet, plate, extrusion, casting etc will all have varied mechanical properties because the forming process effects the directionality in the microstructure.

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6
Q

In products such as sheet and plate, what is the rolling direction called?

A

The longitudinal direction

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7
Q

In products such as sheet and plate, what is the width of the product called?

A

The transverse direction

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8
Q

In products such as sheet and plate, what is the thickness direction called?

A

The short transverse direction

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9
Q

What do mechanical properties of a material change as a function of or are they constant?

A

They change as a function of temperature, rate of loading, and other conditions.

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10
Q

What is loading?

A

The application of a force to an object

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11
Q

What are the 5 fundamental loading conditions?

A

Tension, compression, bending, shear and torsion

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12
Q

What does loading by bending involve?

A

Applying a load in a manner that causes a material to curve and results in compressing the material on one side and stretching it on the other

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13
Q

What does shear loading involve?

A

Applying a load parallel to a plane which causes the material on one side of the plane to want to slide across the material on the other side of the plane.

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14
Q

What does loading by torsion involve?

A

The application of a force that causes twisting in a material

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15
Q

What is static loading?

A

When a material is subjected to a constant force

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16
Q

What is dynamic or cyclic loading?

A

When the loading on the material is not constant but instead fluctuates

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17
Q

What does the term stress express?

A

The loading in terms of force applied to a certain cross-sectional area of an object. Stress is the applied force or system of forces that tends to deform a body.

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18
Q

From the perspective of what is happening within a material, what is stress?

A

The internal distribution of forces within a body that balance and react to the loads applied to it.

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19
Q

What makes a stress uniform or non uniform?

A

The type of loading condition. E.g. a bar loaded in pure tension will have a uniform tensile stress distribution but a bar loaded in bending will have a stress distribution that changed with distance perpendicular to the normal axis.

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20
Q

Why are simplifications and assumptions often made to represent stress as a vector quantity?

A

For engineering calculations and material property determination.

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21
Q

What is a vector?

A

A quantity that has a magnitude and a direction.

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22
Q

Explain the methodical/complex process used to define stress in most 2D or 3D solids?

A

The internal force acting on a small area of a plane can be resolved into three components: one normal to the plane and two parallel to the plane. The normal force component divided by the area gives the normal stress (s), and parallel force components divided by the area give the shear stress (t). These stresses are average stresses as the area is finite, but when the area is allowed to approach zero, the stresses become stresses at a point. Since stresses are defined in relation to the plane that passes through the point under consideration, and the number of such planes is infinite, there appear an infinite set of stresses at a point. Fortunately, it can be proven that the stresses on any plane can be computed from the stresses on three orthogonal planes passing through the point. As each plane has three stresses, the stress tensor has nine stress components, which completely describe the state of stress at a point.

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23
Q

What is strain?

A

Strain is the response of a system to an applied stress. When a material is loaded with a force, it produces a stress, which then causes a material to deform.

24
Q

What is engineering strain?

A

Engineering strain is defined as the amount of deformation in the direction of the applied force divided by the initial length of the material. This results in a unitless number, although it is often left in the unsimplified form, such as inches per inch or meter per meter.

25
Q

What is elastic deformation?

A

If the stress is small, the material may only strain a small amount and the material will return to its original size after the stress is released.

26
Q

What is the stress limit for elastic deformation?

A

When stresses are lower than a critical stress called the yield strength.

27
Q

What are true stress and strain?

A

True stress and strain account for changes in cross-sectional area by using the instantaneous values for area.

28
Q

Why does the engineering stress-strain curve not give a true indication of the deformation characteristics of a metal?

A

Because it is based entirely on the original dimensions of the specimen, and these dimensions change continuously during the testing used to generate the data.

29
Q

Why, if engineering stress and strain aren’t true indications of the behaviour of a material, are they still often used?

A

Because it is easier to generate the data and the tensile properties are adequate for engineering calculations.

30
Q

What is stress concentration phenomenon?

A

When an axial load is applied to a piece of material with a uniform cross-section, the normal stress will be uniformly distributed over the cross-section. However, if a hole is drilled in the material, the stress distribution will no longer be uniform. Since the material that has been removed from the hole is no longer available to carry any load, the load must be redistributed over the remaining material. It is not redistributed evenly over the entire remaining cross-sectional area but instead will be redistributed in an uneven pattern that is highest at the edges of the hole.

31
Q

What are tensile properties?

A

Tensile properties are an indication of how the material will react to forces being applied in tension.

32
Q

What is a tensile test?

A

A fundamental mechanical test where a carefully prepared specimen is loaded in a very controlled manner while measuring the applied load and the elongation of the specimen over some distance.

33
Q

What are tensile tests used for?

A

To determine the modulus of elasticity, elastic limit, elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties.

34
Q

What is the main product of a tensile test?

A

A load vs elongation curve which is then converted into a stress vs strain curve.

35
Q

Draw a typical stress-strain curve with all the parts labelled?

A

see desktop image - stress-strain-curve.png

36
Q

What is the first region of the stress-strain curve called?

A

The linear-elastic region, this is where stress and strain initially increase with a linear relationship and where elastic deformation occurs.

37
Q

What relationship does the linear region of the stress strain curve?

A

Hooke’s law, where the ratio of stress to strain is a constant

38
Q

What is the slope of the line in the linear region of the stress strain curve?

A

This is the modulus of elasticity or the Young’s modulus.

39
Q

What does the modulus of elasticity or Young’s modulus define?

A

The properties of a material as it undergoes stress, deforms, and then returns to its original shape after the stress is removed. It is a measure of the stiffness of a given material (stress/strain). The modulus of elasticity applies specifically to the situation of a component being stretched with a tensile force.

40
Q

What is Poisson’s ratio?

A

The negative of the ratio of the lateral strain to the axial strain for a uni-axial stress state (v = -(strain_lat)/(strain_ax)).

41
Q

What does a Poisson’s ratio of 0.3 mean?

A

If there is one inch per inch of deformation in the direction that stress is applied, there will be 0.3 inches per inch of deformation perpendicular to the direction that force is applied.

42
Q

What is shear modulus?

A

The shear modulus describes the linear-elastic stress-strain relation relationship when a component is subjected to pure shear.

43
Q

Equation for the relationship between the elastic constants E, G and n?

A 
E = 2 * ( 1 + n ) * G
where:
E = Young's modulus
n = Poisson's ratio
G = Shear modulus
44
Q

What is the bulk modulus?

A

Bulk modulus is used to describe the situation where a piece of material is subjected to a pressure increase on all sides. The relationship between the change in pressure and the resulting strain produced is the bulk modulus.

45
Q

What are Lame’s constants?

A

These are constants derived from modulus of elasticity and Poisson’s ratio.

46
Q

What is the yield strength?

A

The yield strength is defined as the stress required to produce a small amount of plastic deformation.

47
Q

What is ultimate tensile strength?

A

The ultimate tensile strength (UTS) is the maximum engineering stress level reached in a tension test.

48
Q

What is the strength of a material?

A

The ability of a material to withstand external forces without breaking.

49
Q

Why in ductile materials is the UTS not the same as the breaking strength?

A

Because strain hardening occurs when the UTS is reached which allows for increased strain while stress starts to decrease. This is a result of engineering stress being based on the original cross-section area and not accounting for the necking that commonly occurs in the test specimen.

50
Q

What is the ductility of a material?

A

A measure of the extent to which a material will deform before fracture (important when considering forming operations such as rolling and extrusion).

51
Q

What is the compression test like compared to the tensile test?

A

The compression test is simply the opposite of the tension test with respect to the direction of loading.

52
Q

Examples of mechanical properties obtained from compression tests?

A

Compressive yield stress, compressive ultimate stress and compressive modulus of elasticity.

53
Q

What is ultimate compressive strength?

A

The stress required to rupture a specimen. This is much harder to determine than for a tensile test since many materials do not exhibit rapid fracture in compression.

54
Q

What is a bearing test for?

A

To determine the deformation of a hole as a function of the applied bearing stress.

55
Q

What is a bearing test like?

A

The test specimen is a piece of sheet or plate with a carefully prepared hole some distance from the edge. A hardened pin is inserted through the hole and an axial load is applied to the specimen and the pin. The bearing stress is computed by dividing the load applied to the pin by the bearing area (pin diameter * plate thickness).

56
Q

What is hardness?

A

The resistance of a material to localized deformation. The term can apply to deformation from indentation, scratching, cutting or bending.

NDT Materials and Processes (3 decks)

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